1. Field of the Invention
The present invention is a variation of a set-up to generate phase contrast X-ray images using special arrangements of gratings. The set-up can be used to record absorption contrast images, phase contrast images, and dark field contrast images of an object. The arrangement thereby improves the visibility of low absorbing specimens and can therefore significantly reduce the required radiation dose without compromising the image quality or provide complimentary image information.
In comparison to existing arrangements in x-ray grating-based imaging systems, the present configuration uses gratings made in a novel planar geometry. This approach has two essential advantages:                (i) it allows for the fabrication of gratings with extreme aspect ratios, making the method particularly useful for high x-ray energies; and        (ii) it can be used to realize grating geometries matched to divergent beam geometries. This arrangement of gratings is particularly suited for a scanning type of x-ray phase contrast imaging, comprising one or several line detectors and a translation of the sample during image acquisition.        
Envisaged applications are for medical scanners (in particular mammography), inspection at industrial production lines, non-destructive testing, and homeland security.
2. Physical Background
It is well known that, differently from conventional visible light optics, the refractive index in X-ray optics is very close to and smaller than unity. In first approximation, for small and negligible anisotropy in the medium, the index of refraction characterizing the optical properties of a tissue can be expressed—including X-ray absorption—with its complex form: n=1−δ−iβ where δ is the decrement of the real part of the refractive index, characterizing the phase shifting property, while the imaginary part β describes the absorption property of the sample. In conventional absorption-based radiography, the X-ray phase shift information is usually not directly utilized for image reconstruction. However, at photon energies greater than 10 keV and for light materials (made up of low-Z elements), the phase shift term plays a more prominent role than the attenuation term because β is typically three orders of magnitude larger than β. As a consequence, phase-contrast modalities can generate significantly greater image contrast compared to conventional, absorption-based imaging. Furthermore, far from absorption edges, δ is inversely proportional to the square of the X-ray energy whilst β decreases as the fourth power of energy. A significant consequence of this mechanism is that phase signals can be obtained with much lower dose deposition than absorption, a very important issue when radiation damage has to be taken into account such as in biological samples or in living systems.
Several approaches have been developed in order to record the phase signal. They can be classified as interferometric methods (with crystals), phase propagation methods, techniques based on an analyzer crystal, or on x-ray gratings. The described invention is in context with the latter technique.
Grating based x-ray imaging setups essentially detect the deflections of x-rays in the object. Such deflections can be either caused by refraction on phase shift gradients in the object resulting in differential phase contrast (DPC) or by scattering on inhomogeneities in the sample resulting in the so-called dark-field image (DFI) contrast. The DPC image signal can be used to obtain phase contrast (PC) images by image processing routines.
Set-ups with two gratings (G1 and G2) or three gratings (G0, G1, and G2) can be applied to record the deflection of the x-rays. In the case of a two-grating set-up, the source needs to fulfill certain requirements regarding its spatial coherence, while in a three grating setup no spatial coherence is required. Therefore, the three grating set-up is suited for use with incoherent x-ray sources, in particular with x-ray tubes.
To separate the conventional attenuation contrast (AC) from the DPC and DFI contrast, a phase-stepping approach is applied. One of the gratings is displaced transversely to the incident beam whilst acquiring multiple images. The intensity signal at each pixel in the detector plane oscillates as a function of the displacement. The average value of the oscillation represents the (AC). The phase of the oscillation can be directly linked to the wave-front phase profile and thus to the DPC signal. The amplitude of the oscillation depends on the scattering of x-rays in the object and thus yields the DFI signal.
For the (two or three) gratings, several approaches have been proposed and applied. The grating G0 (if required) is the one closest to the source. It usually consists of a transmission grating of absorbing lines with the period p0. It can be replaced by a source that emits radiation only from lines with the same period. The grating G1 is placed further downstream of the source. It consists of lines with a period p1. The grating G2 is the one most downstream of the setup. It usually consists of a transmission grating of absorbing lines with the period p2. It can be replaced by a detector system that has a grating-like sensitivity with the same period.
Two regimes of setups can be distinguished: in the so called “near field regime” and the “Talbot regime”. In the “near field regime”, the grating periods p, grating distances d and the x-ray wavelength λ are chosen such, that diffraction effects are negligible. In this case, all gratings need to consist of absorbing lines. In the “Talbot regime”, diffraction on the grating structures is significant. Here G1 should consist of grating lines that are either absorbing or, preferentially, phase shifting. Several amounts of phase shift are possible, preferentially π/2 or multiples thereof. The grating periods must be matched to the relative distances between the gratings. In case of setups in the “Talbot regime” the Talbot effect needs to be taken into account to obtain good contrast.
The sample is mostly placed between G0 of G1 (or upstream of G1 in case of a two-grating set-up), however it can be advantageous to place it between G1 and G2. The presented inventions are relevant in all of the abovementioned cases, i.e. in the two- and three-grating case, in the case of the “nearfield regime” and the “Talbot regime”, and for the sample placed upstream or downstream of G1.
Some commercial x-ray imaging systems use a scanning scheme for imaging. The sample is irradiated with a fan beam, and a line detector and a sample translation are used to acquire a 2-dimensional image of the object. The main advantages of the scheme are, that line detectors are much less expensive than 2D detectors, and that they can be made with higher efficiency, which reduces radiation dose to the sample.
A combination of grating-based x-ray imaging with a scanning setup has been proposed, and experimentally verified (see FIG. 2). This scanning set-up is of particular interest in context of the invention described further below. When a single set of gratings and line detector is used, either the single step approach can be applied, or a phase stepping needs to be done by moving one of the three gratings perpendicular to the grating lines. This phase stepping scan needs to be nested with the object scan, and can thus be very complicated or time consuming. A nested phase stepping and object scan can be avoided by using n fan beams, n sets of grating and n line-detectors. By aligning each of the n sets with a different phase-stepping position, the object will be scanned in n phase-step positions without moving any mechanical parts (besides the object).
The key components of grating-based x-ray imaging are obviously the gratings. Two main technical difficulties are encountered in the fabrication and application of these gratings:
1) The sensitivity of grating based imaging becomes better with decreasing grating periods, which are therefore in the micrometer range (typ. 1-20 microns). On the other hand, the required thickness of the grating lines (i.e. their dimension along the beam path) has to be sufficient to induce enough attenuation (in case of absorbing lines) or sufficient phase shift (in case of phase-shifting gratings). Especially for high x-ray energies, for example above 50 keV, the required grating line thicknesses are usually much higher than the period of the grating lines, resulting in very high aspect ratios. For high x-ray energies, gratings with such high aspect ratios are very difficult, or even impossible, to fabricate.
2) For the use with x-ray tube sources, the image detector size is comparable to the source distance, meaning that the beam has a significant divergence, resulting in a cone-beam geometry, where a 2-dimensional detector is used, and in a fan-beam geometry, where a 1-dimensional (line) detector is used. When the gratings are made on flat substrates with the surface normal along the optical axis (as indicated in FIG. 1), the beams towards the edge of the image field will hit the grating angle in an inclined angle as indicated in FIG. 4. This leads to loss of phase or dark-field contrast, and poses a fundamental problem especially at high x-ray energies where extreme aspect ratios of the grating lines are required. The grating lines would need to be tilted towards the source point, which is difficult to realize for substrates oriented normal to the optical axes. Attempts to bend the substrate or to compose the gratings of smaller pieces, each facing towards the source, have been proposed. However these approaches are technically difficult and expensive.